1. Field of the Invention
The present invention relates to a surface acoustic wave filter, and an antenna duplexer and communication equipment using this surface acoustic wave filter.
2. Related Art of the Invention
FIG. 23 shows a reception frequency band (810 to 828 MHz) and a transmission frequency band (940 to 958 MHz) in a PDC system, a domestic digital cellular telephone system. This configuration employs a method of simultaneously carrying out transmissions and receptions in place of a conventional time division multiple access (TDMA). Thus, this configuration system requires an antenna switch. FIG. 25 is an example of a plan view of a longitudinal coupled mode type surface acoustic wave filter used as a reception filter in a PDC system used in the frequency bands mentioned above.
A surface acoustic wave filter 101, shown in FIG. 25, has IDT electrodes 103 to 105 and reflector electrodes 106 and 107 on a piezoelectric substrate 108. The IDT electrodes 103 to 105 each have a plurality of electrode fingers. An upper bus-bar side of the IDT electrode 103 is connected to a terminal 102. A lower bus bar side of each of the IDT electrodes 104 and 105 is connected to a second terminal 109. The lower bus bar side of the IDT electrode 103 and the upper bus-bar side of each of the IDT electrodes 104 and 105 are grounded.
FIG. 24 shows an attenuation characteristic 111 of the longitudinal coupled mode type surface acoustic wave filter 101 configured as described above. Further, a curve 112, shown in FIG. 24, shows the attenuation characteristic observed in the reception frequency band of the PDC (hereinafter simply referred to as the “reception frequency band”), the attenuation characteristic being enlarged in accordance with the scale on the right axis. As seen in FIG. 24, the attenuation in the transmission frequency band of the PDC (hereinafter simply referred to as the “transmission frequency band”) is about −42 db. Such attenuation is not sufficient, so that transmission signals affect reception signals in an antenna duplexer.
FIG. 26 is a Smith chart showing an impedance characteristic as viewed from the terminal 102 of the surface acoustic wave filter 101. In FIG. 26, reference numeral 113 denotes an impedance characteristic at 950 MHz in the transmission frequency band. Reference numerals 114 and 115 denote impedance characteristics at 810 MHz and 828 MHz, respectively, in the reception frequency band.
FIG. 26 indicates that in the reception frequency band, the impedance characteristics are almost matched to each other but that in the transmission frequency band, the phase is not in its open position. If in the transmission frequency band, the phase is not in its open state, transmission signals may be lost in the antenna duplexer.
Thus, as shown in FIG. 27, the surface acoustic wave resonator 116 is cascaded between the terminal 102 and the surface acoustic wave filter 101. The surface acoustic wave resonator 116 has an IDT electrode 117 and reflector electrodes 118 and 119. The IDT electrode 117 is formed with a plurality of electrode fingers. An upper bus-bar side of the IDT electrode 117 is connected to the terminal 102. A lower bus bar side of the IDT electrode 117 is connected to an upper bus-bar side of the IDT electrode 103 of the surface acoustic wave filter 101.
FIG. 28 shows an attenuation characteristic 120 of the surface acoustic wave resonator 116. Further, a curve 121, shown in FIG. 28, shows this attenuation characteristic enlarged in accordance with the scale on the right axis. As seen in FIG. 28, the surface acoustic wave resonator 116 has an attenuation pole of about −20 dB in the transmission frequency band. Further, this figure indicates that insertion loss amount to about −0.7 dB in reception frequency bands denoted by R1 and R2. The surface acoustic wave resonator is advantageous in attenuating the vicinity of a pass band but insertion loss is degraded at a distance from the pass band. That is, it is preferable that the resonance frequency of the surface acoustic resonator is set near to the pass band of the longitudinal coupled mode type surface acoustic wave filter, and that the anti-resonance frequency of the surface acoustic resonator is set near to the attenuation band of the longitudinal coupled mode type surface acoustic wave filter. (for example, see Japanese Patent Laid-Open No. 6-260876. The entire disclosure of this document is incorporated herein by reference in its entirety). Such a insertion loss in the reception frequency band increases in a communication system such as a PDC system in which the transmission signal frequency band and the reception signal frequency band are separated from each other.
FIG. 29 shows the attenuation characteristics of a surface acoustic wave filter 130 constructed by cascading together the surface acoustic wave resonator 116 having such a characteristic as shown in FIG. 28 and the surface acoustic wave filter 101. The surface acoustic wave filter 130 enables an attenuation pole of about −62 dB to be formed in the transmission frequency band. Consequently, the antenna duplexer can sufficiently attenuate transmission signals in the transmission frequency band compared to a filter composed of the unitary surface acoustic wave filter 101, shown in FIG. 25.
Next, a surface acoustic wave filter 140, shown in FIG. 30, comprises the surface acoustic wave filter 130, shown in FIG. 27, and a surface acoustic wave resonator 131 cascaded to the surface acoustic wave filter 130. The surface acoustic wave resonator 131 has a configuration similar to that of the surface acoustic wave resonator 116 and has an IDT electrode 132 and reflector electrodes 133 and 134. The terminal 102 is connected to an upper bus-bar side of the IDT electrode 132. A lower bus bar side of the IDT electrode 132 is connected to an upper bus-bar side of the IDT electrode 117 of the surface acoustic wave resonator 116.
FIG. 31 shows the attenuation characteristic of the surface acoustic wave filter 140. FIG. 31 indicates that the attenuation in the transmission frequency band is larger. That is, this surface acoustic wave filter achieves much larger attenuation than the surface acoustic wave filter 130, shown in FIG. 27.
However, with the surface acoustic wave filter 130, shown in FIG. 27, a large ripple 200 occurs in the reception frequency band as shown in FIG. 29. Further, with the surface acoustic wave filter 140, shown in FIG. 30, a large ripple 210 occurs in the reception frequency band as shown in FIG. 31.
The presence of such a ripple in the reception frequency band increases insertion loss in this band. This is because owing to the characteristics of the surface acoustic wave resonators 116 and 131, if the transmission frequency band and the reception frequency band are separated from each other as in the case with the PDC, a heavy insertion loss occurs in the reception frequency band as shown in FIG. 28.